Method and apparatus for in situ measurement of corrosion in filled tanks

ABSTRACT

An aspect of the present invention is an apparatus for inspecting a base of liquid filled tank for corrosion, having (a) a housing for use in the liquid filled tank; (b) a set of one or more ultrasonic transducers mounted to the housing, for directing one or more ultrasonic pulses at the base, where the ultrasonic pulses each have a frequency selected to produce a return signal from the base, and for receiving this return signal; and (c) a data capturing system, for storing information from these return signals. Optional features include a second set of one or more ultrasonic transducers for directing one or more ultrasonic pulses at the liquid/gas interface at a frequency selected to produce a return signal from the liquid/gas interface, a data analysis system, a locomotive system, and a spatial location system. Another aspect of the invention is a method for inspecting a base of a liquid filled tank for corrosion, having the steps: (a) directing a broadband ultrasonic pulse at the base from an ultrasonic transducer within the tank, where the ultrasonic pulse includes a resonant frequency for the tank base over the range of expected thicknesses for the base; (b) receiving a return signal with the ultrasonic transducer; (c) performing a Fourier analysis on the return signal to generate a frequency domain signal; and (d) determining the thickness of the base from the frequency domain signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to monitoring liquid storagetanks for corrosion. More particularly, the present invention relates toa novel apparatus and method for the in situ monitoring such tanks forthinning (due to corrosion) of the tank bottom, and optionally foruneven settling of the tank, using ultrasonic transducers mounted on amobile robotic device for use inside liquid filled tanks.

2. Description of the Related Art.

With respect to storage tanks.

Large metal storage tanks are used to store a variety of liquids,including especially water, petroleum, and petroleum products. Thesetanks, which are commonly made from non-stainless steel plates, aresubject to corrosion and eventual failure over time. Even tanks that areintended to store only petroleum and petroleum products almostinevitably will have a substantial amount of water in them, greatlyaggravating their tendency to oxidize. Thus, it is necessary toperiodically inspect these tanks, and to make repairs before suchfailure. However, because of the enormous size of these tanks, it isexpensive and inconvenient to empty a tank each time it is to beinspected.

Typical tanks for petroleum products are made from 1/4" to 1/2" thicksteel plates welded together. They are commonly hundreds of feet talland hundreds of feet in diameter. They are usually built above ground,but may also be made at least partially below ground. If such a tankshould fail while filled with product, the environmental damage would bestaggering.

Complicating the inspection process is the presence of sediment in thetanks. At the bottom of most liquid storage tanks there is a layer ofsediment including rust, dirt, debris, petroleum solids, etc. This layermay be anywhere from several millimeters to several feet thick. Aneffective tank inspection system must either be able to see through thissediment, or must be able to displace the sediment to allow for directinspection.

Bilges of large ships (e.g., aircraft carriers, tankers) likewise mustbe inspected periodically for corrosion. Many of the same concerns areraised for the inspection of bilges as are raised for the inspection oftanks.

With respect to corrosion monitoring systems.

Available systems for monitoring liquid filled tanks for corrosionsuffer from any of several common shortcomings.

One problem that has not been satisfactorily addressed by the art hasbeen the difficulty of using ultrasonic probes to obtain reflectionsignals from both the top and bottom surfaces of the base plate of atank. The sediment layer described above scatters ultrasonic waves,dramatically attenuating these waves, and thereby decreasing theirability to penetrate the bottom steel plate. Moreover, ultrasonic wavesscattered from the sediment are a source of noise that present systemsdo not adequately address.

U.S. Pat. No. 5,205,174, issued Apr. 27, 1993 to Silverman et al.(Silverman '174) is representative. This patent teaches the use ofultrasonic transducers for inspecting the bottom surfaces of storagetanks. This system teaches the use of very high frequency ultrasonicpulses (about 15.4 MHz, with a wavelength of 0.0156'). Such highfrequency pulses will be so diminished in amplitude that signaldetection would be problematic at best. This is particularly true if theultrasonic pulse is scattered by plate surface that has been roughenedby corrosion and attenuated by a layer of sediment. Silverman '174attempts to mitigate the former problem by including a cleaning systemin the apparatus, to remove sediment from the bottom as the apparatusmoves along. In principle, this system would work by scrubbing andvacuuming the base free of sediment, irrigating the base with a streamof clean fluid to remove any remaining sediment, and filtering out thesediment so that it is not returned to the tank. At best, this is a verycomplicated system that does not address the problem of scattering by asurface that has been roughened by corrosion.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide detailedinformation on tank bottom thickness.

It is a further object of this invention to provide such informationeven for heavily corroded liquid storage tanks.

It is a further object of the invention to provide such information evenfor liquid storage tanks having a layer of sediment on the bottom.

These and additional objects of the invention are accomplished by thestructures and processes hereinafter described.

An aspect of the present invention is an apparatus for inspecting a baseof a liquid filled tank for corrosion, having (a) a housing for use inthe liquid filled tank; (b) a set of one or more ultrasonic transducersmounted to the housing, for directing one or more ultrasonic pulses atthe base, where the ultrasonic pulses each have a frequency selected toproduce a return signal from the base, and for receiving this returnsignal; and (c) a data capturing system, for storing information fromthese return signals. Optional features include a second set of one ormore ultrasonic transducers for directing one or more ultrasonic pulsesat the liquid/gas interface at a frequency selected to produce a returnsignal from the liquid/gas interface, a data analysis system, alocomotive system, and a spatial location system.

Another aspect of the invention is a method for inspecting a base of aliquid filled tank for corrosion, having the steps: (a) directing abroadband ultrasonic pulse at the base from an ultrasonic transducerwithin the tank, where the ultrasonic pulse includes a resonantfrequency for the tank base over the range of expected thicknesses forthe base; (b) receiving a return signal with the ultrasonic transducer;(c) performing a Fourier analysis on the return signal to generate afrequency domain signal; and (d) determining the thickness of the basefrom the frequency domain signal.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be obtained readilyby reference to the following Description of the Preferred Embodimentsand the accompanying drawings in which like numerals in differentfigures represent the same structures or elements, wherein:

FIG. 1 shows a schematic of the system according to the invention.

FIG. 2 shows an elevation of a transducer according to the invention.

FIG. 3 shows an elevation of another transducer according to theinvention.

FIG. 4 shows a detailed section of a system according to the invention.

FIG. 5 shows a view of an array of transducers according to theinvention.

FIG. 6 shows a schematic of an electronic system according to theinvention.

FIG. 6A shows a schematic of another electronic system according to theinvention.

FIG. 7 shows a time domain plot of a transducer signal in a methodaccording to the invention.

FIG. 8 shows a frequency domain plot of a gated portion of the timedomain signal depicted in FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With respect to the overall system of the invention.

Referring to FIG. 1, this shows a schematic of the system 10 accordingto the invention. A tank 12 is filled with a liquid inventory 14, suchas a fuel. The tank 12 has a base 13 typically made of welded steelplates. The system of the invention is preferably designed to image thetop 15 and bottom 17 surfaces of the tank base 13. Inside the tank 12 isa remote, automated device 16 housing the ultrasonic transducers (notshown), and the propulsion system 18 (shown here as a motor-driven trackdevice). Also shown (typically inside the tank) are locationtransmitters 20 in communication with the device 16, for locating thedevice 16 within the tank 12. The mobile device 16 is typicallyconnected to a data collection and analysis system 22 (typically aprogrammed digital computer). These connections typically are through atelemetry system (not shown), but alternatively may be through cabling.A microwave power source 24 preferably is used to power the device 16.Alternatively, cabling or large capacity on-board batteries or fuelcells are used to power the device. These power sources may also be usedin conjunction with one another, e.g., batteries used as the principalpower source during operations, and a microwave power source used torecharge the batteries, and also for additional power during operation.

Propulsion systems may be bottom-drive systems (such as the track drivesystem shown), or free-swimming systems, such as propeller or jet drivensystems.

With respect to the transducers of the invention.

Referring to FIG. 2, this shows an elevation of a typical ultrasonictransducer 30 used according to the invention. Typical transducers usedaccording to the invention will have a piezoelectric element 32 (usuallyPZT), mounted in a housing 34 that contains the electrodes coupled tothe piezoelectric element 32, for driving the element. Typically, thehousing 34 also has the contact 37 for electrically connecting thetransducer to the console housing the pulse generator and datacollection and analysis system (not shown).

Referring to FIG. 3, this shows a preferred ultrasonic transducer 38used according to the invention. This transducer has separatepiezoelectric elements for transmitting 40 and receiving 42. The twopiezoelectric elements 40,42 are mounted in a housing 44 that containsthe electrodes coupled to the piezoelectric elements 40,42. Preferably,the two elements are coaxial. Preferably, the receiving element has asmaller diameter than the transmitting element-the smaller diameter willpermit the receiving element to receive a signal integrated over asmaller area, leading to better spatial resolution. Contacts 37,37'electrically connect the two elements to the console housing the pulsegenerator and data collection and analysis system (not shown).

The system of the invention will include at least one set oftransducers, and preferably two sets of transducers (for illustrationpurposes, a single transducer is sometimes shown and referred to hereinas representative of a set of transducers). As shown in FIG. 4, thefirst transducer 38 will be directed toward the base of the tank(typically on the bottom of the housing 19), for imaging the bottomplate of the tank. The optional second transducer 38' will be directedtoward the top of the tank (typically on the top of the housing 19), formeasuring the height of the liquid.

The configurations and operating frequencies of the two transducers neednot be the same, since they perform different functions.

The first transducer 38 should be configured to generate a signal fromthe top 15 and bottom 17 surfaces of the tank base 13. This will entailtransmitting a pulse that can penetrate a highly attenuating layer ofsediment on the tank floor, and reflect a portion of the pulse back offthe top surface. It will further entail transmitting enough of theremaining portion of the pulse to the bottom surface (despite typicalfurther scattering by the corroded top surface of the base andattenuation through the steel plate) so that energy can reflect off thebottom surface of the steel plate. It has been discovered that thefrequencies taught in the art (in particular, the frequencies taught inthe previously cited Silverman '174) are far too high to successfullygenerate a useful return signal off both the corroded top and bottomsurfaces of the steel base plate. It has been discovered thatfrequencies between approximately 0.5 MHz and 2.0 MHz are preferred, andthat a frequency of about 1.0 MHz is more preferred for a base plate ofabout 1/4" thickness. Skilled practitioners will recognize that if thepulse frequency is too low, the spatial resolution of the signal will bereduced beyond a point that is acceptable.

The second transducer 38' should be configured to generate a signal fromthe top surface of the liquid in the tank 14. Because spatial resolutionwill be more critical for imaging the bottom plate than for imaging theliquid surface, using a less expensive single element transducer 30should be acceptable.

Rather than using a single transducer to image in a given direction, itis preferred to use an array of transducers, to provide imaging over awide swath of the tank bottom with each pass of the probe. A simple1-dimensional array of multiplexed transducers will speed up imagingover the tank bottom, for instance. Preferably, as shown in FIG. 5, a2-dimensional array of transducers is used. As shown, an array 39 oftransducers 38 may be arranged so that each transducer will yieldinformation about a given portion of a path of a given width over whichthe array passes.

With respect to the electronic system.

Referring to FIG. 6, this schematically depicts an exemplary systemaccording to the invention. An oscillator 50 provides timing for thesystem. A pulse generator 52 coupled to the oscillator transmitsregularly timed broadband ultrasonic pulses to the transducer. A typicalcommercially available pulse generator will have built-in amplification.In response to a pulse, the transducer transmitter 40 transmits abroadband ultrasonic pulse. The transducer receiver 42 receives a returnsignal, which it transmits to an amplifier 54. The amplifier 54amplifies the return signal, and transmits the amplified return signalto a temporal tracking and spectral analyzer 56, that samples the returnsignal, performs a Fourier analysis on a gated portion of the returnsignal, and transmits the Fourier analysis data to a computer for dataprocessing and image display 58. A synchronization lead 60 sends asynchronization signal to the temporal tracking and spectral analyzer56, to provide synchronization. An interface bus 62, typically a generalpurpose interface bus, connects the computer 58, the temporal trackingand spectral analyzer 56, and the pulse generator 52, to provide datatransfer and control.

A console 22 typically house the oscillator 50,pulse generator 52,computer 58, spectral analyzer 56, and amplifier 54.

Alternatively, as shown in FIG. 6A, remote, automated device 16 housesmost of these electronic components, and microwave transceivers 64 areused for communication between the computer 58 and the device 16.

The optimal gated return signal sampled will depend on the pulse lengthof the transmitted signal and the transmission time through the liquidmedium. It is desired to sample only the portion of the return signalthat does not include the pulse output from the transducer or thereflection from the top surface of the tank base. Morever, the gatedreturn signal should not include the subsequent reflections. It has beenfound that for pulse lengths of about 2 μs, gate lengths that arebetween about two and about ten times the pulse length work well in thepresent invention.

As noted, the peak frequency will be inversely proportional to thethickness of the bottom plate. The relationship between the thicknessand the peak frequency is given by ν=c/λ, where ν is the peak frequency,c is the propagation velocity of the ultrasonic waves in the tank bottommaterial, and λ is the wavelength of the first harmonic, which will beproportional to the tank bottom thickness. However, it may be that theabsolute propagation velocity is not known for a given tank bottom. Ifthe propagation velocity is not known absolutely, then the absolute tankbottom thickness at a given point cannot be determined. However, arelative thickness may be determined. For instance, all the tank bottomthicknesses may be normalized to (expressed as a fraction of) themaximum thickness (corresponding to the point on the tank bottom withthe lowest peak frequency). Typically, this map of the normalized tankbottom thickness will be sufficient to achieve the objects of theinvention. Alternatively, an estimate of the propagation velocity may beused.

With respect to the propulsion system.

A number of propulsion systems are suitable for use in the presentinvention. For example, a track system 18 will work well in most typesof tanks. See Silverman '174, incorporated by reference herein. Othersystems that should work include propulsion systems using propellers andpropulsion systems using liquid jets.

With respect to the positioning system.

Several positioning systems are suitable for use in the presentinvention. As discussed supra, several positioning transmitters may bepositioned within the tank, and the device may triangulate a positionfrom these transmitters, similar to what is done with the globalpositioning system. Alternatively, an inertial navigational systemand/or an active sonar system may be used to monitor the position of thedevice. Temperature correction may be used to enhance the precision ofan active sonar locating system.

With respect to the mapping system.

Thickness data preferably is correlated with positional data collectedfrom the positioning system, to produce a map of the tank bottom. Thismap may be displayed in any suitable manner, e.g., as a color plot withdifferent colors representing different bottom thicknesses, as a contourplot, grayscale plot, or as a three dimensional bar or line plot.

Having described the invention, the following example is given toillustrate specific applications of the invention, including the bestmode now known to perform the invention. This specific example is notintended to limit the scope of the invention described in thisapplication.

EXAMPLE 1

Measurement of Aluminum Plate Thickness for Sample Not Covered bySediment

An aluminum sample having stepped thicknesses ranging from 0.52 cm to1.31 cm was placed in a small tray of water. A custom-built ultrasonictransducer was placed in the tank, and aimed at a portion of thealuminum plate having a known thickness of 0.65 cm, from a distance ofabout 5 cm. The transducer was connected to an analysis unit comprisinga Tektronix DSA602A Digitizing Signal Analyzer, a Metrotek Pulser ModelMP215 (or, in some experiments, a Stanford Research Model DG535 FourChannel Digital Delay/Pulser Generator and ENI Power Amplifier Model325LA), a Metrotek Receiver Model MR101 (or a Matec Broadband ReceiverModel 605), a Metrotek Scanner and Controller Model C403, and aPanametrics 1 MHz broadband transducer (or Valpey-Fisher special order 1MHz broadband annular array transducer), configured as shown in FIG. 6.A broadband ultrasonic pulse having a center frequency ν_(center) =1.0MHz and a bandwidth of approximately from 0.8 MHz to 1.5 MHz (full widthat half maximum) was transmitted to the steel plate. The time domainsignal received by the transducer is shown in FIG. 7. As seen in FIG. 7,the broadband output pulse lasted about 5 μs (from t=0 to about t=about5 μs). The ultrasonic pulse reflected from the front surface of the A1plate, generating a high intensity return signal for about 10 μs, fromabout (from about t=55 μs to about t=65 μs). After about 65 μs, thereturn signal had a reduced intensity. The next 20 μs (from about t=65μs to about t=85 μs) was taken as the resonant return signal, and wasgated for Fourier analysis.

A fast Fourier transform was performed on the gated portion of thereturn signal, with the results shown in FIG. 8. The peak frequency isassociated with the thickness of the plate, as discussed supra.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:
 1. A method for inspecting a base of a liquid filledtank for corrosion, comprising the steps:(a) directing an ultrasonicpulse at a first point on said base from a first ultrasonic transducerwithin said tank, wherein said ultrasonic pulse has a frequency selectedto produce a return signal including reflections from both a top surfaceof said base and a bottom surface of said base; (b) receiving saidreturn signal with said first ultrasonic transducer; (c) performing aFourier analysis on said return signal to generate a first transducerfrequency domain signal; and (d) determining a thickness for said baseat said first point from said first transducer frequency domain signal.2. The method of claim 1, wherein said frequency is between about 0.5MHz and about 2.0 MHz.
 3. The method of claim 1, wherein said frequencyis selected to generate a reflection through said base wherein said basecomprises at least 1/4" steel.
 4. The method of claim 3, wherein saidsteel is non-stainless steel.
 5. The method of claim 4, wherein saidsteel is corroded steel.
 6. The method of claim 1, wherein said firstpoint is at a known position in said tank.
 7. The method of claim 6,wherein said method further comprises the steps of:(e) translating saidfirst ultrasonic transducer to at least one different position withinsaid tank; and (f) at each of said different positions, performing steps(a) through (d) of claim
 1. 8. The method of claim 7, wherein saidmethod further comprises the step of:(g) storing each of saidthicknesses for each of said points as a map of said base.
 9. The methodof claim 1, wherein said method further comprises the steps of:(h)directing an ultrasonic pulse to a first point at a liquid surface froma second ultrasonic transducer within said tank, wherein said ultrasonicpulse has a frequency selected to produce a second transducer returnsignal including a reflection from said liquid surface; (i) receivingsaid return signal with said second ultrasonic transducer; (j) analysingsaid return signal in the time domain, to generate a second transducertime domain signal; and (k) determining a distance from said secondtransducer to said first point at said liquid surface from saidfrequency domain signal.
 10. The method of claim 9, wherein saidfrequency of said pulse from said second transducer is between about 100kHz and about 10 MHz.
 11. The method of claim 9, further comprising thesteps of:(1) determining a distance from said first transducer to saidtop surface of said base at said first point from said first transducer;and (m) determining a liquid height at said first point from saiddistance from said first transducer to said top surface of said base atsaid first point, said distance from said second transducer to saidfirst point at said liquid surface, and a distance between said firsttransducer and said second transducer.
 12. The method of claim 11,further comprising the steps of:(n) translating said first and secondultrasonic transducers to at least one different position within saidtank; (o) at a plurality of said different positions, performing steps(a) through (d) of claim 1, steps (h) through (k) of claim 9, and steps(l) and (m) of claim 11; and (p) storing said liquid heights for each ofsaid points as a map of said liquid heights.
 13. An apparatus forinspecting a base of a liquid filled tank for corrosion, comprising:ahousing adapted for use in said liquid filled tank; a first ultrasonictransducer mechanically coupled to said housing, for directing anultrasonic pulse at said base, wherein said ultrasonic pulse has afrequency selected to produce a return signal including reflections fromboth a top surface of said base and a bottom surface of said base, andfor receiving said return signal; a data capturing system, for storing aplurality of said return signals; and an analysis system adapted forperforming a Fourier analysis on said return signal to generate a firsttransducer frequency domain signal and determining a thickness for saidbase from said first transducer frequency domain signal.
 14. Theapparatus of claim 13 wherein said frequency is between about 0.5 MHzand about 2.0 MHz.
 15. The apparatus of claim 13 further comprising:apropulsion system, for moving said housing to a plurality of pointswithin said tank; and a positioning system, for determining the positionof said housing within said tank.
 16. The apparatus of claim 13 whereinsaid first transducer is a composite transducer having a transmitportion and a receive portion, said transmit and receive portionsarranged concentrically and said receive portion being smaller than saidtransmit portion.
 17. The apparatus of claim 16, wherein said receiveportion is not larger than about 1/4" in diameter.
 18. The apparatus ofclaim 13 further comprising:a second ultrasonic transducer mechanicallycoupled to said housing, for directing an ultrasonic pulse at a liquidsurface, wherein said ultrasonic pulse has a frequency selected toproduce a return signal including a reflection from said liquid surface,and for receiving said return signal; and wherein said data capturingsystem is further adapted for storing a plurality of said return signalsfrom said liquid surface.
 19. The apparatus of claim 18, furthercomprising an analysis system adapted for performing a Fourier analysison said return signals from said first and second ultrasonic transducersto generate a first transducer frequency domain signal and a secondtransducer frequency domain signal, and for determining a thickness forsaid base from said first transducer frequency domain signal and adistance to said liquid surface from said second transducer frequencydomain signal.
 20. The apparatus of claim 13, wherein said firstultrasonic transducer is an element in an array of ultrasonictransducers for directing ultrasonic pulses at said base, wherein saidultrasonic pulses have frequencies selected to produce return signalsincluding reflections from both said top surface of said base and saidbottom surface of said base, and for receiving said return signals. 21.An apparatus for inspecting a base of a liquid filled tank forcorrosion, comprising:means for directing an ultrasonic pulse at a firstpoint on said base from a first ultrasonic transducer within said tank,wherein said ultrasonic pulse has a frequency selected to produce areturn signal including reflections from both a top surface of said baseand a bottom surface of said base; means for receiving said returnsignal with said first ultrasonic transducer; means for performing aFourier analysis on said return signal to generate a first transducerfrequency domain signal; and means for determining a thickness for saidbase at said first point from said first transducer frequency domainsignal.